Polymeric tracers

ABSTRACT

Tracing subterranean fluid flow includes providing a first polymeric tracer to a first injector, collecting a first aqueous sample from a first producer, and assessing the presence of the first polymeric tracer in the first aqueous sample. The first polymeric tracer includes a first polymer formed from at least a first monomer. The presence of the first polymeric tracer in the first aqueous sample is assessed by removing water from the first aqueous sample to yield a first dehydrated sample. pyrolyzing the first dehydrated sample to yield a first gaseous sample, and assessing the presence of a pyrolization product of the first polymer in the first gaseous sample. The presence of the pyrolization product of the first polymer in the first gaseous sample is indicative of the presence of a first subterranean flow pathway between the first injector location and the first producer location.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of and claims the benefit of priorityto U.S. patent application Ser. No. 15/800,886, filed Nov. 1, 2017,which claims the benefit of U.S. Provisional Application Ser. No.62/418,433 entitled “POLYMERIC TRACERS” and filed on Nov. 7, 2016, bothof which are incorporated by reference herein in their entirety.

TECHNICAL FIELD

This document relates to thermal decomposition of polymeric tracers andtheir detection via pyrolysis gas chromatography-mass spectrometry(GCMS) for oilfield tracer applications.

BACKGROUND

Hydrocarbon bearing reservoirs are complicated, interconnectedsubterranean systems containing multiple fluid phases. Understanding theconnectivity between injection and production wells is a factor inefficient reservoir management as unexpected early water breakthrough ata given well can drastically reduce the oil production rate. Tracerstudies provide a means to understand how water is being allocated inthe subsurface and to inform the reservoir engineer how to adjustinjection rates to mitigate water production. Traditionally, tracerstudies are performed by injecting water soluble molecules such asfluorinated benzoic acids and fluorescent dyes such as rhodamine orfluorescein, followed by identification at the producing well via massspectrometry or fluorescence, respectively. This information may be usedto build a map of the fluid pathways in the subsurface environment.

Reservoir simulations of inter-well tracer diffusion over time havedemonstrated that tracers injected at different wells may be extractedfrom a single producer, highlighting the importance of clear andunambiguous tracer signals to aid in distinction. This poses variousproblems for standard fluorescence-based tracers, which generateoverlapping signals given their wide bandwidths of emission (50 nm to100 nm). This problem, compounded with the limited range of fluorescencedetection (300 nm to 1200 nm), the background fluorescence of crudepetroleum, and the decline of quantum yield and detectability at higherwavelengths, reduces the amount of distinguishable tracers even forsmall reservoirs with a moderate number of wells. Moreover, sincefluorescence is sensitive to the local environment, salinity,temperature, and the presence of dissolved organic matter makequantitation difficult.

There are other drawbacks to using molecular tracers in the oilfield aswell. Due to their small size, molecules tend to diffuse to a greaterextent within the matrix as compared to larger entities such asparticles, polymers and dendrimers. This leads to lower concentrationsat the producing well and greater difficulty in detection. Moleculartracers have to be isolated from the aqueous producing fluid becausewater is not compatible with gas chromatograph-mass spectrometry (GCMS)instrumentation. This is time consuming and expensive. Each uniquemolecular tracer has to be vetted for reservoir applications byverifying that the proposed tracer does not stick to the reservoirmatrix, is thermally stable and uniquely identifiable. Satisfying all ofthese specifications drastically reduces the number of potential tracersthat could be used. Thus, there is a need to develop an alternativeplatform to molecular tracers that permits the development of a richbarcoding scheme for elucidating connectivities in complicated,interconnected subterranean systems containing multiple fluid phases.

SUMMARY

The disclosed systems and methods relate to detection of tracermaterials in saline, aqueous matrices, the capability to barcode amultiplicity of tracers, and wellhead detection capability. Overall,tracer campaigns serve to inform reservoir managers of subsurface flowpatterns. This information, in turn, is used to better allocate thefluids to maximize oil recovery.

Thermal decomposition of polymeric tracers provides a method to generatemany tens or hundreds of polymeric barcodes which can be unambiguouslyidentified through the use of pyrolysis-GCMS. The sharp andquasi-discrete signals offered by GCMS detectors make them suitable forconvoluted inter-well tracer analysis. As an added benefit, GCMSdetection is suitable for tracers that differ in mass, thereforeeliminating demand for expensive functionalized molecular taggants usedin fluorescence and other tracing systems, reducing material costs, andincreasing the number of tags suitable for tracing.

This approach also eliminates reliance post harvesting extractionprotocols, as the pyrolyzer can be used to remove any water orinterferrents from the system. Electrolytes are mitigated due to theirextremely high boiling points which are not accessed during pyrolysis.In addition, the disclosed processes have superior atom economy, in thatthe entire tracer mass contributes to detectable signals.

In a first general aspect, tracing fluid flow in a subterraneanformation includes providing a first polymeric tracer to a firstinjector location in the subterranean formation, collecting a firstaqueous sample from a first producer location in the subterraneanformation, and assessing the presence of the first polymeric tracer inthe first aqueous sample. The first polymeric tracer includes a firstpolymer formed from at least a first monomer. The presence of the firstpolymeric tracer in the first aqueous sample is assessed by removingwater from the first aqueous sample to yield a first dehydrated sample,pyrolyzing the first dehydrated sample to yield a first gaseous sample,and assessing the presence of a pyrolization product of the firstpolymer in the first gaseous sample. The presence of the pyrolizationproduct of the first polymer in the first gaseous sample is indicativeof the presence of the first polymeric tracer in the first aqueoussample, and the presence of the first polymeric tracer in the firstaqueous sample is indicative of the presence of a first subterraneanflow pathway between the first injector location and the first producerlocation.

Implementations of the first general aspect may include one or more ofthe following features.

In some cases, the pyrolization product of the first polymer includesthe first monomer. In certain cases, the pyrolization product of thefirst polymer is the first monomer.

In some cases, the pyrolization product of the first polymer includes asubstituent on the first monomer. In certain cases, the pyrolizationproduct of the first polymer is a substituent on the first monomer.

Some embodiments include forming a diagrammatic representation ofsubterranean fluid flow in the subterranean formation. The diagrammaticrepresentation typically includes the first injector location, the firstproducer location, and an indicator of the first subterranean flowpathway if the pyrolization product of the first polymer is present inthe first gaseous sample.

Certain embodiments include providing a second polymeric tracer to asecond injector location, collecting a second aqueous sample from thefirst producer location, and assessing the presence of the secondpolymeric tracer in the second aqueous sample. The second polymerictracer includes a second polymer formed from at least a second monomer.The second monomer differs in mass from the first monomer, and thesecond aqueous sample differs from the first aqueous sample. Assessingthe presence of the second polymeric tracer in the second aqueous sampleincludes removing water from the second aqueous sample to yield a seconddehydrated sample, pyrolyzing the second dehydrated sample to yield asecond gaseous sample, and assessing the presence of a pyrolizationproduct of the second polymer in the second gaseous sample. The presenceof the pyrolization product of the second polymer in the second gaseoussample is indicative of the presence of the second polymeric tracer inthe second aqueous sample, and the presence of the second polymerictracer in the second aqueous sample is indicative of the presence of asecond subterranean flow pathway between the second injector locationand the first producer location.

In some cases, the pyrolization product of the second polymer includesthe second monomer. In certain cases, the pyrolization product of thesecond polymer is the second monomer.

In some cases, the pyrolization product of the second polymer includes asubstituent on the second monomer. In certain cases, the pyrolizationproduct of the second polymer is a substituent on the second monomer.

Some embodiments include forming a diagrammatic representation ofsubterranean fluid flow in the subterranean formation. The diagrammaticrepresentation typically includes the first injector location, thesecond injector location, the first producer location, an indicator ofthe first subterranean flow pathway if the pyrolization product of thefirst polymer is present in the first gaseous sample, and an indicatorof the second subterranean flow pathway if the pyrolization product ofthe second polymer is present in the second gaseous sample.

Some embodiments include providing a second polymeric tracer to a secondinjector location and assessing the presence of the second polymerictracer in the first aqueous sample. The second polymeric tracer includesa second polymer formed from at least a second monomer. The secondmonomer differs in mass from the first monomer. Assessing the presenceof the second polymeric tracer in the first aqueous sample includesassessing the presence of a pyrolization product of the second polymerin the first gaseous sample. The presence of the pyrolization product ofthe second polymer in the first gaseous sample is indicative of thepresence of the second polymeric tracer in the first aqueous sample, andthe presence of the second polymeric tracer in the first aqueous sampleis indicative of the presence of a second subterranean flow pathwaybetween the second injector location and the first producer location.The presence of the first subterranean flow pathway and the secondsubterranean flow pathway is indicative of fluid connectivity betweenthe first subterranean flow pathway and the second subterranean pathway.

Certain embodiments include forming a diagrammatic representation ofsubterranean fluid flow in the subterranean formation. The diagrammaticrepresentation typically includes the first injector location, thesecond injector location, the first producer location, an indicator ofthe first subterranean flow pathway if the pyrolization product of thefirst polymer is present in the first gaseous sample, an indicator ofthe second subterranean flow pathway if the pyrolization product of thesecond polymer is present in the first gaseous sample, and an indicatorof the fluid connectivity between the first subterranean pathway and thesecond subterranean pathway if the pyrolization product of the firstpolymer and the pyrolization product of the second polymer are bothpresent in the first gaseous sample.

In some cases, a length of time between providing the first polymerictracer and collecting the first aqueous sample is in a range of days toyears.

In some cases, the pyrolization product of the first polymer is notpresent in the first gaseous sample. In these cases, embodiments includecollecting a second aqueous sample from the first producer location inthe subterranean formation, and assessing the presence of the firstpolymeric tracer in the second aqueous sample. A length of time betweenproviding the first polymeric tracer and providing the second polymerictracer may be in a range of days to years.

In one example, the first polymeric tracer includes a first polymericnanoparticle. The first polymeric nanoparticle may include a polymericcoating over a polymeric core, where the polymeric core comprises thefirst polymer. In some examples, the first polymer includes polystyrene,polyacrylate, polymethacrylate, or a vinyl polymer.

Some embodiments include removing water from the first aqueous sample.Removing water from the first aqueous sample may include heating thefirst aqueous sample for a first length of time at a first temperaturegreater than the boiling point of water, where the degradationtemperature of the first polymer is greater than the first temperature.In some examples, the first temperature is in a range of 200° C. to 400°C. In some examples, the first length of time in a range of 10 secondsto 2 minutes, the second length of time is in a range of 10 seconds to 2minutes, or both.

Some embodiments include pyrolyzing the first dehydrated sample.Pyrolyzing the first dehydrated sample may include heating the firstdehydrated sample for a length of time to a temperature greater than thedegradation temperature of the first polymer.

In some embodiments, assessing the presence of the pyrolization productof the first polymer in the first gaseous sample includes providing thefirst gaseous sample to a gas chromatograph to yield an output includingcomponents of the first gaseous sample. Assessing the presence of thepyrolization product of the first polymer in the first gaseous samplemay include providing the output of the gas chromatograph to a detector.In some examples, the detector includes a mass spectrometer or a flameionization detector.

In some embodiments, the first aqueous sample includes saline.

Some embodiments include collecting a second aqueous sample from asecond producer location in the subterranean formation, and assessingthe presence of the first polymeric tracer in the second aqueous sample.Assessing the presence of the first polymeric tracer in the secondaqueous sample may include removing water from the second aqueous sampleto yield a second dehydrated sample, pyrolyzing the second dehydratedsample to yield a second gaseous sample, and assessing the presence ofthe pyrolization product of the first polymer in the second gaseoussample. The presence of the pyrolization product of the first polymer inthe second gaseous sample is usually indicative of the presence of thefirst polymeric tracer in the second aqueous sample, and the presence ofthe first polymeric tracer in the second aqueous sample is usuallyindicative of the presence of a second subterranean flow pathway betweenthe first injector location and the second producer location.

In a second general aspect, a polymeric tracer includes a polymericcore, a polymeric layer surrounding the polymeric core, and a surfactantlayer between the polymeric core and the polymeric layer. The polymericcore includes a polymer that thermally depolymerizes above thedegradation temperature of the polymer into one or more pyrolizationproducts.

Implementations of the second general aspect may include one or more ofthe following features.

The polymeric tracer may be a nanoparticle. In some cases, the polymeris formed from at least one of a polystyrene, a polyacrylate, apolymethacrylate, or a vinyl monomer. In certain cases, the polymer is acopolymer. The one or more pyrolization products may include one or moreconstituent monomers of the polymer. In one example, at least one of oneor more constituent monomers is the reaction product of a vinylanilineand an epoxide. In another example, the surfactant layer includes sodiumdodecyl sulfate. In yet another example, the polymeric layer includespolyethyleneimine.

In a third general aspect, a polymeric tracer library includes amultiplicity of polymeric tracers, each polymeric tracer comprising apolymeric core, and each polymeric core comprising a polymer thatthermally depolymerizes into one or more pyrolization products. Eachpyrolization product of the multiplicity of polymeric tracers differs inmolecular mass from the other pyrolization products of the multiplicityof polymeric tracers.

Implementations of the third general aspect may include one or more ofthe following features.

Each polymeric tracer may be a nanoparticle. In some embodiments, atleast one pyrolization product of each polymer includes or is aconstituent monomer of that polymer. In certain embodiments, at leastone pyrolization product of each polymer includes or is a substituent ofa constituent monomer of that polymer.

The details of one or more implementations of the subject matterdescribed in this specification are set forth in the description below.Other features, aspects, and advantages of the subject matter willbecome apparent from the description and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a polymeric tracer.

FIG. 2 depicts depolymerization of a polymer at a temperature greaterthan its degradation temperature.

FIG. 3 is a flow chart for a process for tracing subterranean fluid flowfrom a first injector location to a first producer location in asubterranean formation.

FIG. 4 is a flow chart for a process for assessing the presence of apolymeric tracer in an aqueous sample.

FIG. 5 is a flow chart for a process for tracing subterranean fluid flowfrom a second injector location to a first producer location in asubterranean formation.

FIG. 6 depicts a diagrammatic representation of subterranean fluid flowfrom a first injector location and a second injector location to a firstproducer location in a subterranean formation.

FIG. 7 is a flow chart for a process for tracing subterranean fluid flowfrom a first injector location and a second injector location to a firstproducer location in a subterranean formation.

FIG. 8 is a flow chart for a process for assessing the presence of afirst polymeric tracer and a second polymeric tracer in an aqueoussample.

FIG. 9 depicts a diagrammatic representation of subterranean fluid flowfrom a first injector location and a second injector location to a firstproducer location in a subterranean formation.

FIG. 10 depicts a diagrammatic representation of subterranean fluid flowfrom a first injector location and a second injector location to a firstproducer location and a second producer location in a subterraneanformation.

FIG. 11 shows retention times of monomers resulting from pyrolysis ofpolymeric tracers.

FIGS. 12A-12C show pyrograms of copolymeric nanoparticlescross-referenced to pyrograms of their constituent monomers.

FIGS. 13 and 14 show peak intensity analysis of monomer mixtures.

FIG. 15 shows pyrolysis temperature stages on a single sample ofsurfactant.

FIGS. 16A and 16B show pyrograms of polymer coated nanoparticles inseawater with and without crude petroleum, respectively.

FIG. 17 shows pyrograms of surfactant, polymeric tracers, and polymerictracers with crude petroleum.

FIG. 18 shows pyrograms of polymeric tracers, each with a distinctmonomer.

FIG. 19 shows total ion chromatograms (TICs) after pyrolysis ofpolymeric tracers in aqueous matrices.

FIG. 20 shows a total ion chromatogram (TIC) after pyrolysis of amixture of styrene and 4-tert-butylstyrene nanoparticles.

DETAILED DESCRIPTION

Reference will now be made in detail to certain embodiments of thedisclosed subject matter. While the disclosed subject matter will bedescribed in conjunction with the enumerated claims, it will beunderstood that the exemplified subject matter is not intended to limitthe claims to the disclosed subject matter.

Values expressed in a range format should be interpreted in a flexiblemanner to include not only the numerical values explicitly recited asthe limits of the range, but also to include all the individualnumerical values or sub-ranges encompassed within that range as if eachnumerical value and sub-range is explicitly recited. For example, arange of “about 0.1% to about 5%” or “about 0.1% to 5%” should beinterpreted to include not just about 0.1% to about 5%, but also theindividual values (for example, 1%, 2%, 3%, and 4%) and the sub-ranges(for example, 0.1% to 0.5%, 1.1% to 2.2%, 3.3% to 4.4%) within theindicated range. The statement “about X to Y” has the same meaning as“about X to about Y,” unless indicated otherwise. Likewise, thestatement “about X, Y, or about Z” has the same meaning as “about X,about Y, or about Z,” unless indicated otherwise.

In this document, the terms “a,” “an,” or “the” are used to include oneor more than one unless the context clearly dictates otherwise. The term“or” is used to refer to a nonexclusive “or” unless otherwise indicated.The statement “at least one of A and B” has the same meaning as “A, B,or A and B.” In addition, it is to be understood that the phraseology orterminology employed in this disclosure, and not otherwise defined, isfor the purpose of description only and not of limitation. Any use ofsection headings is intended to aid reading of the document and is notto be interpreted as limiting; information that is relevant to a sectionheading may occur within or outside of that particular section.

In the methods of manufacturing, the acts can be carried out in anyorder, except when a temporal or operational sequence is explicitlyrecited. Furthermore, specified acts can be carried out concurrentlyunless explicit claim language recites that they be carried outseparately. For example, a claimed act of doing X and a claimed act ofdoing Y can be conducted simultaneously within a single operation, andthe resulting process will fall within the literal scope of the claimedprocess.

The term “about” can allow for a degree of variability in a value orrange, for example, within 10%, within 5%, or within 1% of a statedvalue or of a stated limit of a range.

The term “about” refers to a majority of, or mostly, as in at leastabout 50%, 60%, 70%, 80%, 90%, 95%, 96%, 97%, 98%, 99%, 99.5%, 99.9%,99.99%, or at least about 99.999% or more.

The phrase “degree of polymerization” is the number of repeating unitsin a polymer.

The term “polymer” refers to a molecule having at least one repeatingunit and can include copolymers.

The term “copolymer” refers to a polymer that includes at least twodifferent repeating units. A copolymer can include any suitable numberof repeating units.

The term “fluid” refers to gases, liquids, gels, and critical andsupercritical materials.

The phrase “subterranean formation” refers to any material under thesurface of the earth, including under the surface of the bottom of theocean. For example, a subterranean formation can be any section of awellbore and any section of a subterranean petroleum- or water-producingformation or region in fluid contact with the wellbore. Placing amaterial in a subterranean formation can include contacting the materialwith any section of a wellbore or with any subterranean formation influid contact with a section of a wellbore. In some examples, asubterranean formation can be any below-ground region that can produceliquid or gaseous petroleum materials, water, or any sectionbelow-ground in fluid contact with a below-ground region that canproduce liquid or gaseous petroleum materials or water. For example, asubterranean formation can be at least one of an area desired to befractured, a fracture or an area surrounding a fracture, and a flowpathway or an area surrounding a flow pathway. A fracture or a flowpathway can be optionally fluidly connected to a subterranean petroleum-or water-producing region, directly or through one or more fractures orflow pathways.

A “flow pathway” downhole can include any suitable subterranean flowpathway through which two subterranean locations are in fluidconnection. The flow pathway can be sufficient for petroleum or water toflow from one subterranean location to the wellbore or vice-versa. Aflow pathway can include at least one of a hydraulic fracture, and afluid connection across a screen, across gravel pack, across proppant,including across resin-bonded proppant or proppant deposited in afracture, and across sand. A flow pathway can include a naturalsubterranean passageway through which fluids can flow. In someembodiments, a flow pathway can be a water source and can include water.In some embodiments, a flow pathway can be a petroleum source and caninclude petroleum. In some embodiments, a flow pathway can be sufficientto divert from a wellbore, fracture, or flow pathway connected theretoat least one of water, a downhole fluid, or a produced hydrocarbon.

Polymeric Tracers

Polymeric tracers are detectable tracers that are suitable for tracingsubterranean fluid flow in a subterranean formation. Polymeric tracersmay be used to provide a multiplicity of unique tracers for large scaletracer campaigns, thereby permitting multi-well interrogationsimultaneously. For example, if a field contains multiple injectors andproducers, each injector may be treated with a unique polymeric tracerin order to know with certainty which injector is communicating withwhich producer. It is often the case that multiple injectors will becommunicating with a single producer, thereby complicating the situationfurther. The disclosed polymeric tracers permit the development of arich barcoding scheme for large scale tracer campaigns.

Polymeric tracers include molecular structures that have a multiplicityof monomers held together by covalent bonds and that thermally decomposeto yield smaller fragments, such as the monomers, substituents on themonomers, or other fragments that can be detected in the gas phase.Suitable molecular structures include water-soluble polymers (includingwater-soluble copolymers), water-soluble dendrimers, polymericnanoparticles, and the like.

The term “nanoparticle” generally refers to a particle having a largestdimension of up to 100 nm. The nanoparticle may be a coatednanoparticle. In some embodiments, the nanoparticles or coatednanoparticles have a particle size or average size of 10 nm to 100 nm.The term “average size” generally refers to the arithmetic mean of thedistribution of nanoparticle sizes in a plurality of nanoparticles. Insome examples, the nanoparticles or coated nanoparticles can have aparticle size or average size of 20 nm to 80 nm, 30 nm to 50 nm, or lessthan 100 nm. Nanoparticle size can be determined by dynamic lightscattering prior to forming a coated nanoparticle or by scanningelectron microscopy after formation of a coated nanoparticle. In someembodiments, a coated nanoparticle has a hydrodynamic diameter of 10 nmto 100 nm. For example, a coated nanoparticle can have a hydrodynamicdiameter of 20 nm to 80 nm, 30 nm to 50 nm, or less than 100 nm. Thedegree of polymerization n of a polymer in a polymeric tracer may be ina range of 5 to 1,000,000.

FIG. 1 depicts a cross section of exemplary polymeric tracer 100. Insome implementations, polymeric tracer 100 is a polymeric nanoparticle.Polymeric tracer 100 typically includes polymeric core 102 and one ormore coating layers surrounding the polymeric core. As depicted in FIG.1, polymeric tracer 100 is coated with surfactant layer 104 andpolymeric layer 106.

Polymeric core 102 includes a polymer formed from at least a firstmonomer. In some cases, the polymer is formed from a single monomer. Incertain cases, the polymer is a copolymer formed from two or moremonomers, each monomer having a different mass. A suitable polymer forthe polymeric core depolymerizes into its constituent monomer ormonomers, substituents on the monomers and the polymer backbone, orother fragments at temperatures greater than its degradationtemperature. The “degradation temperature” of a polymer is generallyunderstood to be the temperature at which the rate of polymerization anddepolymerization of the polymer are equal. The degradation temperatureof a polymer is typically in range from 200° C. to 1000° C.

Polymers may depolymerize by various mechanisms, based at least in parton relative bond strengths present in the polymer. Monomer reversionoccurs when bonds created during polymerization are among the weakestbonds in the polymer, and the polymer thermally degrades to yield themonomer(s) from which the polymer is formed. Examples of polymers thatdepolymerize by monomer reversion include styrenes and methacrylates. Inside-chain scission, substituents on the monomer are among the weakestbonds in the polymer, and these bonds are broken when the polymer isheated to yield the substituents and the polymer backbone. An example ofa polymer that depolymerizes by side-chain scission is poly(vinylchloride). In random scission, bond strengths in the polymer differ solittle that thermal degradation of the polymer results in a range ofproducts. Examples of polymers that depolymerize by random scissioninclude polyethylene and polypropylene.

FIG. 2 depicts an example of monomer reversion of a polymer at atemperature greater than the degradation temperature of the polymer toyield the constituent monomer of the polymer, where R is a substituent.In some examples, R is halogen, alkyl, alkoxy, aminoalkyl, orperfluoroalkyl. Polymers that depolymerize into their constituentmonomer or monomers at a temperature greater than the degradationtemperature include polystyrenes, polyacrylates, polymethacrylates, andvinyl polymers formed from styrenic, acrylic, methacrylic, and vinylmonomers, respectively. Although FIG. 2 depicts styrenic and acrylicmonomers having a single substituent, other suitable monomers are not solimited. Examples of suitable styrenic monomers include methylstyrene(such as 4-methylstyrene, α-methylstyrene), methoxystyrene (such as4-methoxystyrene), dimethylstyrene (such as 2,4-dimethylstyrene),trimethylstyrene (such as 2,4,6-trimethylstyrene), halostyrenes (such as4-chlorostyrene, 4-fluorostyrene and 4-bromostyrene), 4-acetoxystyrene,4-benzhydrylstyrene, 4-benzyloxy-3-methoxystyrene, 4-tert-butoxystyrene,2,6-dichlorostyrene, 2,6-difluorostyrene, 3,4-dimethoxystyrene,2,4-dimethylstyrene, 4-ethoxystyrene, pentafluorophenyl 4-vinylbenzoate,2,3,4,5,6-pentafluorostyrene, 4-(trifluoromethyl)styrene,4-vinylbenzocyclobutene, 4-chloromethylstyrene, 4-vinylbiphenyl,4-vinylbenzoic acid, 1,1-diphenylethylene,3,5-bis(trifluoromethyl)styrene, 4-vinylphenyl acetate,trimethoxy(4-vinyl-phenyl)silane, 4-vinylaniline,4-(aminomethyl)styrene, 4-isopropenylaniline,1-(4-vinylphenyl)-ethanamine, 1-(4-vinylphenyl)cyclopropanamine,(1S)-1-(3-vinylphenyl)-1,2-ethanediamine, and 4-vinylphenol.

Examples of suitable epoxides include 1,2-epoxybutane, 1,2-epoxypentane,1,2-epoxyhexane, 1,2-epoxyoctane, 1,2-epoxydodecane,1,2-epoxytetradecane, 1,2-epoxyhexadecane, 2-hexadecyloxirane, allylglycidyl ether, butyl glycidyl ether, tert-butyl glycidyl ether,3,4-epoxy-1-butene, 1,2-epoxy-5-hexene, 1,2-epoxy-9-decene,4-chlorophenyl glycidyl ether, 1,2-epoxy-3-phenoxypropane,(2,3-epoxypropyl)benzene, 2-ethylhexyl glycidyl ether, furfuryl glycidylether, glycidyl hexadecyl ether, glycidyl isopropyl ether, glycidyl4-methoxyphenyl ether, glycidyl 2-methylphenyl ether, glycidyl2,2,3,3,4,4,5,5-octafluoropentyl ether,2,3-epoxy-1-(1-ethoxyethoxy)propane, 1,2-epoxydecane,1,2-epoxyoctadecane, 1,2-epoxyeicosane,2,2,3,3,4,4,5,5,5-nonafluoropentyloxirane,2,2,3,3,4,4,5,5,6,6,7,7,7-tridecafluoroheptyloxirane,1,2-epoxy-1H,1H,2H,3H,3H-heptadecafluoroundecane, glycidyl methyl ether,ethyl glycidyl ether, epichlorohydrin, glycidyl propargyl ether,glycidyl lauryl ether, tert-butyldimethylsilyl (S)-glycidyl ether,3-glycidyloxypropyltrimethoxysilane,3-glycidyloxypropyl(dimethoxy)methylsilane,[8-(glycidyloxy)-n-octyl]trimethoxysilane,triethoxy(3-glycidyloxypropyl)silane,diethoxy(3-glycidyloxypropyl)methylsilane,1,1,1,3,5,5,5-heptamethyl-3-(3-glycidyloxypropyl)trisiloxane,3-[2-(perfluorohexyl)ethoxy]-1,2-epoxypropane, benzyl glycidyl ether,4-tert-butylphenyl glycidyl ether, 2,4-dibromophenyl glycidyl ether,(S)-glycidyl titryl ether, (S)—N-glycidylphthalimide, and4-glycidyloxycarbazole.

Suitable monomers may also be synthesized by utilizing Reaction 1 (shownbelow) between vinylaniline and any epoxide to form a styrenic monomerthan could in turn be used for nanoparticle synthesis. In addition,variants of vinylaniline that undergo Reaction 1 could also be used.

The polymeric core may be formed in a monomer starved addition process.Monomer starved addition is a type of emulsion polymerization. Inemulsion polymerization, a surfactant is used to dispersenon-water-soluble monomer in an aqueous phase, and an initiator is addedto initiate polymerization inside the micelle monomer droplets. Inmonomer starved addition, monomer is provided at a rate less than thepolymerization rate. This allows control of the resulting particle size,since there is no excess monomer and thus no opportunity for anyparticle to grow faster than another or for any droplets to form.

Polymeric core 102 is coated with surfactant layer 104. Surfactant layer104 serves to stabilize monomer droplets during polymerization, and toprovide an electrostatic charge on the periphery of the nanoparticle,thereby facilitating coating with an additional polymer of oppositecharge. Suitable surfactants include anionic surfactants such as alkylsulfates, fatty alcohol ether sulfates, alkyl phenol ether sulfates,poloxamers, alkyl ammonium halides, and others generally known in theart of emulsion polymerization. In some examples, suitable surfactantsinclude cetyl trimethyl ammonium bromide and sodium dodecyl sulfate.

Surfactant layer 104 is coated with polymeric layer 106. Polymeric layer106 serves as a protective coating for polymeric tracer 100. Polymericlayer 106 is substantially inert to aqueous and dried downhole fluidphases including a range of organic and inorganic compounds, such thatpolymeric core 102 is preserved under saline and thermally stressfulreservoir conditions. Polymeric layer 106 may include one or morepolymers, each polymer including one or more monomers. In one example,polymeric layer 106 includes polyethyleneimine (PEI). Other suitablepolymers for polymeric layer 106 include crosslinked carbohydrate-basedcoatings and coatings formed from a linker, a crosslinker, and astabilizing group.

A crosslinked carbohydrate-based coating can include a carbohydrateincluding a monosaccharide, an oligosaccharide, a polysaccharide, andmixtures thereof. In some embodiments, the polysaccharide is selectedfrom the group consisting of an alginate, a chitosan, a curdlan, adextran, a derivatized dextran, an emulsan, agalactoglucopolysaccharide, a gellan, a glucuronan, anN-acetyl-glucosamine, an N-acetyl-heparosan, a hyaluronic acid, akefiran, a lentinan, a levan, a mauran, a pullulan, a scleroglucan, aschizophyllan, a stewartan, a succinoglycan, a xanthan, a diutan, awelan, a starch, a derivatized starch, a tamarind, a tragacanth, a guargum, a derivatized guar gum (for example, a hydroxypropyl guar, acarboxy methyl guar, or a carboxymethyl hydroxypropyl guar), a gumghatti, a gum arabic, a locust bean gum, a cellulose, and a derivatizedcellulose (for example, a carboxymethyl cellulose, a hydroxyethylcellulose, a carboxymethyl hydroxyethyl cellulose, a hydroxypropylcellulose, or a methyl hydroxy ethyl cellulose). In some embodiments,the polysaccharide is dextran.

The polysaccharide can have a number average molecular weight of about1,000 MW to about 150,000 MW. For example, the polysaccharide can have anumber average molecular weight of about 10,000 MW to about 140,000 MW,about 30,000 MW to about 130,000 MW, 50,000 MW to about 120,000 MW,70,000 MW to about 110,000 MW, or about 80,000 MW to about 100,000 MW orabout 1,000 MW, 5,000 MW, 10,000 MW, 20,000 MW, 30,000 MW, 40,000 MW,50,000 MW, 60,000 MW, 70,000 MW, 80,000 MW, 90,000 MW, 100,000 MW,110,000 MW, 120,000 MW, 130,000 MW, 140,000 MW, or about 150,000 MW orgreater.

The polysaccharide can be dextran with a number average molecular weightof about 1,000 MW to about 150,000 MW. For example, the dextran can havea number average molecular weight of about 10,000 MW to about 140,000MW, about 30,000 MW to about 130,000 MW, 50,000 MW to about 120,000 MW,70,000 MW to about 110,000 MW, or about 80,000 MW to about 100,000 MW orabout 1,000 MW, 5,000 MW, 10,000 MW, 20,000 MW, 30,000 MW, 40,000 MW,50,000 MW, 60,000 MW, 70,000 MW, 80,000 MW, 90,000 MW, 100,000 MW,110,000 MW, 120,000 MW, 130,000 MW, 140,000 MW, or about 150,000 MW orgreater.

In some embodiments, the crosslinked carbohydrate-based coating is thereaction product of a crosslinking reaction between an epoxide-basedcompound and a carbohydrate.

Crosslinking the carbohydrate-based coating can promote association ofthe carbohydrate based coating with the underlying nanoparticle. Theepoxide-based compound can be selected from the group consisting ofpolyethylene glycol diglycidyl ether, epichlorohydrin, 1,4-butanedioldiglycidyl ether, ethylene glycol diglycidyl ether, 1,6-hexanedioldiglycidyl ether, propylene glycol diglycidyl ether, poly(propyleneglycol)diglycidyl ether), poly(tetramethylene glycol)diglycidyl ether,neopentyl glycol diglycidyl ether, polyglycerol polyglycidyl ether,diglycerol polyglycidyl ether, glycerol polyglycidyl ether,trimethylpropane polyglycidyl ether,1,2-(bis(2,3-epoxypropoxy)ethylene), pentaerythritol glycidyl ether,pentaerythritol polyglycidyl ether, sorbitol polyglycidyl ether, andmixtures thereof. In some embodiments, the epoxide-based compound ispentaerythritol glycidyl ether.

The crosslinked carbohydrate-based coating can be the reaction productof a quenching reaction between the crosslinked carbohydrate-basedcoating and an amine-functionalized compound. Quenching the crosslinked,carbohydrate based coating can involve reacting an amine with unreactedepoxides present in the crosslinked, carbohydrate-based coating.Additionally, quenching the unreacted epoxides can serve to preventundesired crosslinking between nanoparticles. The amine-functionalizedcompound can have the structure:

The variable R′, at each occurrence, can be independently selected from—H, —OH, or a substituted or unsubstituted (C₁-C₁₀) hydrocarbyl. Forexample, the variable R¹ can be independently selected from —H, —OH, or—(C₁-C₁₀) alkyl-OH. In some embodiments, the amine-functionalizedcompound is 2-amino-2-hydroxymethyl-propane-1,3-diol.

In a coating formed from a linker, a crosslinker, and a stabilizinggroup, the linker may be crosslinked with the crosslinker. Thestabilizing group may be covalently bound to the linker. In someembodiments, the linker is crosslinked with the crosslinker and thelinker is covalently bound to the stabilizing group.

In some embodiments, the linker includes the subunit:

At each occurrence, the variable R¹ can be independently selected fromthe from the group consisting of —H,

or a linear or branched (C₁-C₂₀) alkyl interrupted with 0, 1, 2, 3, 4,5, 6, 7, 8, or 9 substituted or unsubstituted nitrogen atoms, where thewavy line labeled 1 indicates a point of attachment to another linker onthe crosslinked-coated nanoparticle. At each occurrence, the variable Acan be independently selected from a (C₁-C₁₀) alkyl interrupted with 0,1, 2, 3, or 4 oxygen atoms or substituted or unsubstituted nitrogenatoms.

In some embodiments, the linker includes a terminal group, wherein theterminal group is selected from the group consisting of OR^(A) ₂,—SR^(A) ₂, —N—NR^(A) ₂, O—NR^(A) ₂, or NR^(A) ₂. At each occurrence, thevariable R^(A) is independently selected from —H or

where the wavy line labeled 2 can indicate a point of attachment to thestabilizing group.

In some embodiments, the linker includes polyethylenimine.

The crosslinker can include an epoxide functional group. In someembodiments, the crosslinker is a bis-epoxide. The bis-epoxide can be adiglycidyl ether. The diglycidyl ether can be selected from the groupconsisting of 1,4-butanediol diglycidyl ether, poly(ethylene glycol)diglycidyl ether, neopentyl glycol diglycidyl ether, glycerol diglycidylether, 1,4-cyclohexanedimethanol diglycidyl ether, resorcinol diglycidylether, poly(propylene glycol) diglycidyl ether, bisphenol A diglycidylether, diglycidyl ether (C₆H₁₀O₃), 1,2-propanediol diglycidyl ether,1,4-butanediyl diglycidyl ether, and combinations thereof. In someembodiments, the diglycidyl ether includes a 1,4-butanediol diglycidylether.

The stabilizing group can include one or more of —OH, —CO₂H, —CO₂CH₃,phosphate, or sulfate. For example, the stabilizing group can include afunctional group selected from the group consisting of:

and combinations thereof.

In some implementations, a polymeric tracer library includes amultiplicity of polymeric tracers, each polymeric tracer including oneor more polymers suitable for depolymerizing above its degradationtemperature into one or more constituent monomers, substituents on themonomers, or other fragments, where each constituent monomer, monomersubstituent, or other fragment of the multiplicity of polymeric tracersdiffers in molecular mass from the other constituent monomers,substituents on the monomers, or other fragments of the multiplicity ofpolymeric tracers.

FIG. 3 is a flow chart showing a process 300 for tracing a subterraneanfluid flow in a subterranean formation using polymeric tracers. In oneexample, the subterranean formation is a hydrocarbon bearing reservoir.In 302, a first polymeric tracer is provided to a first injectorlocation in the subterranean formation. The first polymeric tracerincludes a first polymer formed from at least a first monomer. In 304,after an elapsed time on the order of days, weeks, months, or years, afirst aqueous sample is collected from a first producer location in thesubterranean formation. The phrase “aqueous sample” generally refers toa volume of liquid that includes water. The water may include one ormore salts, such as sodium chloride. The aqueous sample may include oneor more organic compounds that are immiscible with water. In 306, thepresence of the first polymeric tracer in the first aqueous sample isassessed.

FIG. 4 is a flow chart for a process 400 for assessing the presence ofthe first polymeric tracer in the first aqueous sample. In 402, water isremoved from the first aqueous sample to yield a first dehydratedsample. Removing water from the first aqueous sample includes heatingthe first aqueous sample to a temperature greater than the boiling pointof water and less than the degradation temperature of the first polymerin the first polymeric tracer for a length of time to removesubstantially all of the water from the first aqueous sample. In someexamples, the first aqueous sample is heated in a range of 200° C. to500° C., 300° C. to 500° C., or 350° C. to 450° C. for a length of timein a range of 10 sec to 2 min to yield a first dehydrated sampleincluding the first polymeric tracer. When the first aqueous sampleincludes saline, the first dehydrated sample typically includes saltcrystals. The first polymeric tracer may be adsorbed on the saltcrystals.

In 404, the first dehydrated sample is pyrolyzed to yield a firstgaseous sample. Pyrolyzing the first dehydrated sample includes heatingthe first aqueous solution to a temperature greater than the degradationtemperature of the first polymer and less than the boiling point ofsodium chloride for a length of time. Electrolytes are mitigated due totheir high boiling points which are not accessed during pyrolysis. Inone example, sodium chloride crystals remain unaffected by thepyrolysis, and thus do not interfere with the subsequent analysis of thefirst gaseous sample. In some cases, the first dehydrated sample isheated to a temperature of at least 100° C., at least 200° C., or atleast 300° C. greater than the degradation temperature of the firstpolymer. In one example, the first dehydrated sample is heated to atemperature in a range of 600° C. to 1000° C., or 700° C. to 900° C.Pyrolyzing the first dehydrated sample depolymerizes the first polymerinto pyrolization products including its constituent monomer(s) (thatis, the first monomer), substituent(s) on the monomer(s), or otherfragments in gaseous form. The gaseous sample also includes degradationproducts of the surfactant layer and the polymeric layer. This pyrolysisallows for detection of tracer materials in saline, aqueous matriceswithout extracting the tracer materials from the matrices, as thepyrolyzer can be used to remove any water or interferrents from thefirst aqueous sample.

The depolymerization process in 404 yields low molecular weightcompounds that are amenable to detection by methods such as massspectrometry (MS) and Fourier transform infrared spectroscopy (FTIR), orby separation via gas chromatography (GC) and subsequent detection by MSor flame ionization detection (FID), Fourier transform infraredspectroscopy (FTIR), and the like. The polymeric core of a polymerictracer is not compatible with gas chromatography due at least in part tothe high molecular weight and low volatility of the polymer.Depolymerization of the polymeric core allows detection of the presenceof the polymer in the polymeric core via detection of the monomerconstituent of the polymer.

In 406, the presence of a pyrolization product of the first polymer,such as the first monomer, a substituent on the first monomer, or otherfragment in the first gaseous sample is assessed. Assessing the presenceof the pyrolization product of the first polymer in the first gaseoussample includes providing the first gaseous sample to a gaschromatograph to yield an output including the components of the firstgaseous sample, and providing the output of the gas chromatograph to adetector. In one example, the detector is a mass spectrometer. In otherexamples, the detector is a flame ionization detector, a thermalconductivity detector, or a FTIR detector. The presence of thepyrolization product of the first polymer in the first gaseous sample isindicative of the presence of the first polymeric tracer in the firstaqueous sample, and the presence of the first polymeric tracer in thefirst aqueous sample is indicative of the presence of a firstsubterranean flow pathway between the first injector location and thefirst producer location. Thus, if the pyrolization product of the firstpolymer is present in the first gaseous sample, the first injectorlocation demonstrates fluid connectivity with the first producerlocation.

By using thermally immolative polymers, the polymeric core can beefficiently decomposed and volatilized into pyrolization products(constituent monomers, substituents on the constituent monomers, orother fragments) upon heating beyond the degradation temperature of thepolymer, which can be accomplished using a pyrolysis-detector system inwhich a pyrolyzer is coupled to a detector. The pyrolyzer allowspreheating of an aqueous sample including polymeric tracers to volatizeunwanted background components and vent the resulting degradationproducts to waste before pyrolysis of the polymeric tracers.

The polymeric tracers and associated pyrolization products (“mass tags”)demonstrate (i) tag preservation under saline and thermally stressfulreservoir conditions, (ii) non-degrading conversion of tags into thegaseous phase for GC compatibility, (iii) adequate tag delivery to thedetector after fluid production, and (iv) elimination of backgroundsignals from seawater and crude oil.

FIG. 5 is a flow chart showing a process 500 for tracing a subterraneanfluid flow in the subterranean formation described with respect to FIG.3, in which a second polymeric tracer is provided to a second injectorlocation, and the presence of the second polymeric tracer in a secondaqueous sample from the first producer location is assessed. In 502, asecond polymeric tracer is provided to a second injector location in thesubterranean formation. The second polymeric tracer includes a secondpolymer formed from at least a second monomer that differs in mass fromthe first monomer. In 504, a second aqueous sample is collected from thefirst producer location in the subterranean formation. The secondaqueous sample may be different than the first aqueous sample. In 506,the presence of the second polymeric tracer in the second aqueous sampleis assessed. Assessing the presence of the second polymeric tracer inthe second aqueous sample may be achieved as described with respect toFIG. 4 for assessing the presence of the first polymeric tracer in thefirst aqueous sample.

FIG. 6 depicts a diagrammatic representation of subterranean fluid flowin subterranean formation 600 having first injector location 602, secondinjector location 604, and first producer location 606. A subterraneanflow pathway between first injector location 602 and first producerlocation 606 is depicted by the dashed line between the first injectorlocation and the first producer location. If the pyrolization product ofthe first polymer is present in the first gaseous sample (that is, ifthe first polymeric tracer is present in the first aqueous sample), thena subterranean flow pathway exists between first injector location 602and first producer location 606. If the pyrolization product of thefirst polymer is not present in the first gaseous sample (that is, ifthe first polymeric tracer is not present in the first aqueous sample),then a subterranean flow pathway has not been demonstrated between firstinjector location 602 and first producer location 606. If thepyrolization product of the second polymer is present in the secondgaseous sample (that is, if the second polymeric tracer is present inthe second aqueous sample), then a subterranean flow pathway existsbetween second injector location 604 and first producer location 606. Ifthe pyrolization product of the second polymer is not present in thesecond gaseous sample (that is, if the second polymeric tracer is notpresent in the second aqueous sample), then a subterranean flow pathwayhas not been demonstrated between second injector location 604 and firstproducer location 606.

FIG. 7 is a flow chart for a process 700 for tracing subterranean fluidflow in a subterranean formation using polymeric tracers. In 702, afirst polymeric tracer is provided to a first injector location in thesubterranean formation. The first polymeric tracer includes a firstpolymer formed from at least a first monomer. In 704, a second polymerictracer is provided to a second injector location. The second polymerictracer includes a second polymer formed from at least a second monomer,and the second monomer differs in mass from the first monomer. In 706, afirst aqueous sample is collected from a first producer location. In708, the presence of the first polymeric tracer and the second polymerictracer in the first aqueous sample is assessed.

FIG. 8 is a flow chart for a process 800 for assessing the presence ofthe first polymeric tracer and the second polymeric tracer of FIG. 7 inthe aqueous sample. Aspects of process 800 may be understood withrespect to process 400 as discussed with respect to FIG. 4. In 802,water is removed from the first aqueous sample to yield a firstdehydrated sample. Removing water from the first aqueous sample includesheating the first aqueous sample to a temperature greater than theboiling point of water and less than the degradation temperature of thefirst polymer in the first polymeric tracer and the second polymer inthe second polymeric tracer for a length of time to remove substantiallyall of the water from the first aqueous sample.

In 804, the first dehydrated sample is pyrolyzed to yield a firstgaseous sample. Pyrolyzing the first dehydrated sample includes heatingthe first dehydrated sample to a temperature greater than thedegradation temperature of the first polymer and the second polymer fora length of time. Pyrolyzing the first dehydrated sample depolymerizesthe first polymer and the second polymer into their pyrolizationproducts (for example, the first monomer and the second monomer orsubstituents of the first or second monomers or other fragments) ingaseous form.

In 806, the presence of the pyrolization products of the first andsecond polymers in the first gaseous sample is assessed. Assessing thepresence of the pyrolization products of the first and second polymersin the first gaseous sample may include providing the first gaseoussample to a gas chromatograph to yield an output including thecomponents of the first gaseous sample, and providing the output of thegas chromatograph to a detector. In one example, the detector is a massspectrometer. In another example, the detector is a flame ionizationdetector. The presence of the pyrolization product of the first polymerin the first gaseous sample is indicative of the presence of the firstpolymeric tracer in the first aqueous sample, and the presence of thepyrolization product of the second polymer in the first gaseous sampleis indicative of the presence of the second polymeric tracer in thefirst aqueous sample. The presence of the first polymeric tracer in thefirst aqueous sample is indicative of the presence of a firstsubterranean flow pathway between the first injector location and thefirst producer location. The presence of the second polymeric tracer inthe first aqueous sample is indicative of the presence of a secondsubterranean flow pathway between the second injector location and thefirst producer location. Thus, if the pyrolization product of the secondpolymer is present in the first gaseous sample, the second injectorlocation has fluid connectivity with the first producer location. Thepresence of the pyrolization products of the first and second polymersin the first gaseous sample is indicative of a first subterranean flowpathway between the first injector location and the first producerlocation and a second subterranean flow pathway between the secondinjector location and the first producer location, as well as fluidconnectivity between the first subterranean flow pathway and the secondsubterranean flow pathway.

FIG. 9 depicts a diagrammatic representation of subterranean fluid flowin subterranean formation 900 having first injector location 902, secondinjector location 904, and first producer location 906. A subterraneanflow pathway between first injector location 902 and first producerlocation 906 is depicted by the dashed line between first injectorlocation and the first producer location. If the first monomer ispresent in the first gaseous sample (that is, if the first polymerictracer is present in the first aqueous sample), then a subterranean flowpathway exists between first injector location 902 and first producerlocation 906. If the pyrolization product of the first polymer is notpresent in the first gaseous sample (that is, if the first polymerictracer is not present in the first aqueous sample), then a subterraneanflow pathway has not been demonstrated between first injector location902 and first producer location 906. If the pyrolization product of thesecond polymer is present in the gaseous sample (that is, if the secondpolymeric tracer is present in the aqueous sample), then there is asubterranean flow pathway between second injector location 904 and firstproducer location 906. If the pyrolization product of the second polymeris not present in the gaseous sample (that is, if the second polymerictracer is not present in the aqueous sample), then a subterranean flowpathway has not been demonstrated between second injector location 904and first producer location 906. Fluid connectivity between firstinjector location 902 and second injector location 904 is indicated bythe dashed line between the first injector location and the secondinjector location.

In some implementations of process 300 or process 700, a second aqueoussample is collected from a second producer location in the subterraneanformation, and the presence of the first polymeric tracer in the secondaqueous sample is assessed. Assessing the presence of the firstpolymeric tracer in the second aqueous sample may be understood withrespect to process 400 of FIG. 4, and includes removing water from thesecond aqueous sample to yield a second dehydrated sample, pyrolyzingthe second dehydrated sample to yield a second gaseous sample, andassessing the presence of the pyrolization product of the first polymerin the second gaseous sample. The presence of the pyrolization productof the first polymer in the second gaseous sample is indicative of thepresence of the first polymeric tracer in the second aqueous sample, andthe presence of the first polymeric tracer in the second aqueous sampleis indicative of the presence of a second subterranean flow pathwaybetween the first injector location and the second producer location.

FIG. 10 depicts a diagrammatic representation of subterranean fluid flowin subterranean formation 1000 having first injector location 1002,second injector location 1004, first producer location 1006, and secondproducer location 1008. A subterranean flow pathway between firstinjector location 1002 and first producer location 1006 is depicted bythe dashed line between the first injector location and the firstproducer location. If a pyrolization product of the first polymer ispresent in the gaseous sample (that is, if the first polymeric tracer ispresent in the aqueous sample), then a subterranean flow pathway existsbetween first injector location 1002 and first producer location 1006.If a pyrolization product of the first polymer is not present in thefirst gaseous sample (that is, if the first polymeric tracer is notpresent in the aqueous sample), then a subterranean flow pathway has notbeen demonstrated between first injector location 1002 and firstproducer location 1006. If a pyrolization product of the second polymeris present in the gaseous sample (that is, if the second polymerictracer is present in the aqueous sample), then a subterranean flowpathway exists between second injector location 1004 and first producerlocation 1006. If a pyrolization product of the second polymer is notpresent in the gaseous sample (that is, if the second polymeric traceris not present in the aqueous sample), then a subterranean flow pathwayhas not been demonstrated between second injector location 1004 andfirst producer location 1006. If a pyrolization product of the firstpolymer is present in the second gaseous sample (that is, if the firstpolymeric tracer is present in the second aqueous sample), then asubterranean flow pathway exists between first injector location 1004and second producer location 1008. If a pyrolization product of thefirst polymer is not present in the second gaseous sample (that is, ifthe first polymeric tracer is not present in the second aqueous sample),then a subterranean flow pathway has not been demonstrated between firstinjector location 1002 and second producer location 1008. In some cases,there is fluid connectivity between first injector location 1002 andsecond injector location 1004 with respect to first producer location1006, second producer location 1008, or both.

In some implementations, the presence of a polymeric tracer in aqueoussamples from a producer location may be assessed over time to establishflow characteristics of the subterranean formation based, for example,on elapsed time between providing the polymeric tracer to the injectorlocation and collecting an aqueous sample including the polymeric tracerfrom a producer location. Thus, while a particular polymeric tracer maynot be present in a first aqueous sample from a first producer location,that polymeric tracer may be present in a second aqueous sample from thefirst producer location collected after the first aqueous sample.

In some implementations, a multiplicity of polymeric tracers may beused, each including a monomer, substituent on a monomer, or fragmenthaving a mass distinct from that of the others and provided to one of amultiplicity of injector locations, thereby creating a library orbarcoding scheme elucidating connectivities in complicated,interconnected subterranean systems. The polymeric core of a polymerictracer may include a copolymer having two or more constituent monomers,substituents on the monomers, or fragments, each having a differentmass. The commercial availability of styrenic, acrylic, methacrylic, andvinyl monomers affords opportunities in barcoding as each monomer has aunique molecular mass and thus a unique fingerprint for variousdetection methods. The approach also takes advantage of atom economy, inthat every atom in the nanoparticle is contributing to signal, therebyincreasing the detectability of the polymeric tracers. Moreover, withthe disclosed process, the presence of polymeric tracers can be detectedwithout any interference from water or salt. This approach can also beused to detect other materials in the reservoir, including polymers forwaterflooding, surfactants for enhanced oil recovery, drillingchemicals, scale inhibitors, corrosion inhibitors, polymeric wasteintrinsic to the field water, heavy fractions of crude oil, and thelike.

In some implementations, processes 400 and 800 can be automated by usingone or more computer-executable programs that are executed using one ormore computing devices. In one example, processes 400 and 800 can beexecuted using one or more computing devices to control removing waterfrom an aqueous sample, pyrolyzing a dehydrated sample, or both,including selecting temperatures and duration for drying the aqueoussample and pyrolyzing the dehydrated sample.

EXAMPLES Example 1

Polymeric nanoparticles having a styrenic core coated in a layer ofsurfactant (sodium dodecyl sulfate—SDS) were prepared The nanoparticleswere formed using monomer starved addition synthesis in an airtightflask held at 90° C. in an oil bath. 60 mL of SDS (2.55% in deionizedwater) was added to the flask and left to degas with N₂ for 15 minutes.Next, 100 mg of radical initiator [2,2′-azobis(2-methylpropionamidine)dihydrochloride] was added to the flask and left to dissolve. Afterdissolution of the initiator, a syringe containing the styrenic monomerwas connected through a tube to the flask, and 1 mL of monomer solutionwas injected at 0.02 mL/min using a programmable syringe pump.

The pyrolyzer used was an AS 5250 Pyrolysis Autosampler from CDSAnalytical. It was chosen for the dual purpose of pre-heating andselective thermal volatilization to reach each nanoparticle'sdegradation temperature. Samples were housed in a thin closed-end tubefilled with quartz wool. Liquid samples (usually 1 μL) were injectedinto the wool and held by capillary forces, whereas solids were heldbetween two beds of wool. Tubes were manually loaded into thepyrolyzer's autosampler, which dropped samples into its heatingcompartment, which was then purged with inert gas (helium). Thepyrolyzer then operated in two successive heating stages: drying andpyrolysis. After purging, the exhaust valve shifted to a transfer lineto waste, and the sample was dried at a temperature and duration set bythe operator, allowing for the removal of unwanted materials thatvolatilize at temperatures below the pyrolysis temperature. Afterdrying, the valve shifted to a transfer line (held at 300° C.) to the GCcolumn, and the sample was heated to the desired pyrolysis temperature.The pyrolyzer was configured to run programmed temperature stages on asingle tube, allowing one to separate the signals of chemicalsvolatilizing different temperatures.

The GCMS device used was the G908 (beta version) from 908 Devices.Volatilized material from the pyrolyzer was separated by retention timein the GC column. The GC method places the column on a temperature rampfrom 40° C. to 300° C. at 1° C./min, and then holds it at 300° C. for 90sec for a total run time of 6 minutes. The output flow of material fromthe column is then split between two detectors: the MS and the FID.

Six monomers were purchased for nanoparticle synthesis based on theirchemical structures and octanol-water partition coefficients (quantifiedby predicted log P values). After synthesis, the average particle sizesand concentrations were determined using dynamic light scattering (DLS)and thermogravimetric analysis (TGA), respectively. In addition to thesix nanoparticles above, three other nanoparticles with copolymericcores were also synthesized and characterized. Table 1 lists the sixselected monomers with selected characterization parameters.

TABLE 1 Selected monomers with characterization of synthesizednanoparticles. Average Particle Molecular Concentration DiameterMonomer(s) in core Weight log P (TGA) (DLS) p-methylstyrene 118.18 3.167034 ppm  8.57 nm p-methoxystyrene 134.18 2.64 8570 ppm 44.29 nm2,4-dimethylstyrene 132.20 3.62 7940 ppm 13.62 nm 2,4,6-trimethylstyrene146.23 4.08 7050 ppm 12.90 nm 4-chlorostyrene 138.59 3.22 7880 ppm 14.47nm 4-bromostyrene 183.05 3.59 7660 ppm 12.12 nm p-methyl + p-methoxy — —9830 ppm 91.76 nm 2,4-dimethyl + 4-bromo — — 6490 ppm 11.63 nm4-chloro + 4-bromo — — 7180 ppm 12.61 nm

The results, summarized in Table 1, show that the synthesis reaction hadappreciable yields for each of the monomers given that allconcentrations were greater than 6000 ppm. The average particle diameterfor most solutions was found to be less than 50 nm, which is suitablefor porous transport within a reservoir. Despite similar syntheses, theaverage diameter of particles containing p-methoxystyrene is notablylarger than those of other particles. This is thought to be caused bythe monomer's stronger partitioning into water as indicated by itsrelatively lower log P value.

A sample of SDS was first analyzed with pyrolysis-GCFID (Py-GCFID) inorder to determine the expected background in each of the nanoparticlesolutions. Then each of the nanoparticle solutions was analyzed usingthe same pyrolysis method, in which samples were dried at 300° C. for 20sec and pyrolyzed at 800° C. for 15 sec. FIG. 11 shows FID results frompyrolyzing each nanoparticle solution showing distinct signals ofvarying retention times. Results are compared to the background signalof SDS fragments identified by a triplet peak at 1.45 min and a solitarypeak near 2 min.

The background signal of SDS fragments, which is present in allnanoparticle pyrograms given their SDS coating, is characterized by atriplet peak near 1.45 min and a single peak near 2 min. All monomersyielded distinct and reproducible time-separated signals, allowing foreach to be identified based on retention time. Table 2 shows a list ofretention times for each of the six monomers.

TABLE 2 Monomer retention times. Monomer Retention time (min)p-methylstyrene 0.95 4-chlorostyrene 1.1 2,4-dimethylstyrene 1.2p-methoxystyrene 1.3 4-bromostyrene 1.33 2,4,6-trimethylstyrene 1.36

The copolymeric nanoparticle solutions were analyzed using the samepyrolysis method, and their pyrograms were compared to those of theirconstituent monomers. Ignoring the SDS background, each of thecopolymeric particles produced two-peak signals at retention timessimilar to each of its constituent monomers. FIGS. 12A-12C showpyrograms of copolymeric nanoparticles, cross-referenced to pyrograms oftheir constituent monomers, as the bottom curve of each graph. FIGS.12A-12C show pyrograms for chloro/bromo, dimethyl/bromo, andmethyl/methoxy, respectively. The ability to detect details ofcomposition allows for the use of copolymeric nanoparticles as newdistinct tags—significantly increasing the amount of distinguishabletags from a given set of monomers.

To further test the ability of Py-GC to detect details of samplecomposition, five volumetric mixtures of the 4-chlorostyrene and4-bromostyrene nanoparticle solutions were prepared—each with adifferent Cl/Br ratio. The solutions were pyrolyzed using the previouslydescribed method, and the intensities of the 4-chlorostyrene and4-bromostyrene peaks were monitored.

FIGS. 13 and 14 show peak intensity analysis of volumetric mixtures of4-chlorostyrene and 4-bromostyrene. The results demonstrate a linearresponse of peak intensity to changing proportions of 4-chlorostyrenenanoparticles. Minor deviations from linear behavior can be explained bythe unequal density of monomer units in nanoparticles of different type.The suitable detection resolution allows for these proportional monomermixtures (whether volumetric or copolymeric) to function as additionaldistinct tags.

The SDS background is pervasive in all Py-GC/FID measurements on thenanoparticles, which could complicate tag detection in the future if acertain tag is retained as long as the SDS fragments, so itsvolatilization was studied by exposing a sample of SDS to a set oftemperature stages in the pyrolyzer. The temperature of maximum SDS losswould then be used as the drying temperature for all future pyrolysisruns to ensure the removal of all non-polymeric material beforeanalysis.

The pyrograms at all temperature stages are overlaid in FIG. 15, whichshows pyrolysis temperature stages on a single sample of SDS. Each curverepresents the amount of material volatilizing at its associatedtemperature after having been subjected to the previous temperaturestage for 30 sec. Between 300° C. and 400° C., the peak intensitiesdropped by more than a factor of four to near-negligible levels. At 500°C., the SDS signal is almost completely eliminated; however, 500° C. istoo close to the degradation temperature of most types of polystyrene,so setting it as the drying temperature could compromise monomer signalsand harm limits of detection. Hence, 400° C. was chosen as theappropriate drying temperature and was used for all subsequent pyrolysisruns.

To simulate reservoir conditions, nanoparticle detection was to beattempted in the presence of seawater and crude petroleum. First,aliquots of the p-methylstyrene and the 2,4-dimethylstyrene nanoparticlesolutions were separately coated with polyethyleneimine (PEI) tocolloidally stabilize them in seawater. The coating process compoundedwith the introduction to seawater would dilute the particleconcentrations to approximately 250-350 ppm, so a large decrease in peakintensity is to be expected. First, a sample of PEI in seawater (SW) wasseparately pyrolyzed to determine the background signal. Then PEI-coatedp-methylstyrene nanoparticles in seawater were pyrolyzed, followed bythe pyrolysis of the same solution but with 100 μL of crude Hawiyahpetroleum (Oil) mixed in, further followed by the pyrolysis of the samesolution with additional PEI-coated 2,4-dimethylstyrene nanoparticles.The results are summarized in FIGS. 16A and 16B, which shows pyrogramsof PEI-coated nanoparticles in seawater with and without crudepetroleum.

The top panel in FIG. 16A shows the background signal (PEI+SW). Thebottom panel in FIG. 16A (PEI+p-Methyl+SW) demonstrates the detection ofp-methylstyrene tags in seawater and in the presence of PEI as shown bythe peak at 0.95 min. The top panel in FIG. 16B (PEI+p-Methyl+SW+Oil)demonstrates the detection of the same tag despite the presence of crudepetroleum, and the bottom panel in FIG. 16B(PEI+p-Methyl+2,4-Dimethyl+SW+Oil) demonstrates the detection ofmultiple tags as shown by the additional peak at 1.2 min attributed to2,4-dimethylstyrene. This shows that tag signal would be hardlyaffected, if at all, by reservoir conditions when the sample is dried at400° C. to remove unwanted hydrocarbons before pyrolysis.

To further test the temporal resolution of Py-GC detection, a volumetricmixture of all six nanoparticle solutions was prepared and pyrolyzed(with drying at 300° C.), and the resulting program was compared tothose of the individual nanoparticles. Afterward, 100 μL of crudepetroleum was introduced to the mixture, which was pyrolyzed twoseparate times: once dried at 300° C., and once dried at 400° C., toobserve the effects on the SDS signal. FIG. 17 shows, from top tobottom, pyrograms of: SDS, a volumetric mixture of all 6 nanoparticlesolutions dried at 300° C., nanoparticle mixture with 100 μL of Hawiyahcrude petroleum dried at 300° C., and nanoparticle mixture with 100 μLof Hawiyah crude petroleum dried at 400° C.

Not counting the SDS triplet peak, the second panel from the top in FIG.17 (All NPs) demonstrates 6 distinguishable peaks, each at a retentiontime corresponding to an individual monomer signal. FIG. 18cross-references these peaks with constituent signals. The second panelfrom the bottom in FIG. 17 demonstrates the effects, or lack thereof, ofadding crude petroleum to the mixture. The bottom panel in FIG. 17demonstrates a significant reduction of the SDS triplet peak,effectively clearing the sample space for additional tags if needed.

Example 2

Two additional crosslinked polymer nanoparticle systems were synthesizedusing monomers of varying masses—namely, styrene and t-butylstyrene. Theresulting nanoparticles were dispersed in either deionized water orsynthetic seawater to demonstrate that pyrolysis GCMS can be used todetect the polymeric particles in aqueous matrices. The pyrolyzer wasprogrammed to heat the sample to 200° C. for two minutes in order to ridthe sample of water followed by heating to 700° C. to decompose thepolymeric particle into monomers. The monomers were then analyzed viaGCMS to determine their molecular weight.

Styrenic monomers (styrene 99% stabilized (Acros Organics) and4-tert-butylstyrene 90% stabilized (TCI America)) were purified prior touse by passing the monomer liquid through a short column of basicaluminum oxide for removal of polymerization inhibitor. The polymericparticles were synthesized via a monomer starved approach. In thismethod, a 100 mL three necked flask equipped with a magnetic stir barwas charged with 50 mL of a 3 wt % solution of IGEPAL CA-897 (Solvay) indeionized water (Milli-Q System, Millipore, USA, 18.2 Me). The vesselwas sealed and degassed for 30 minutes by bubbling N₂ through thesolution. After degassing, 40 mg of 2,2′-azobis(2-methylpropionamidine)dihydrochloride 98% (Acros Organics) was added to the solution under anN₂ purge. The pH was adjusted to −9 using 1M NaOH (aq). The solution wasthen heated to 80° C. using an oil bath. During the heating stage, a 10mL syringe was loaded with approximately 3 mL of a 10:1 mixture byvolume of the purified styrenic monomer: purified p-divinyl-benzene 85%(Sigma-Aldrich), and loaded onto a Harvard Apparatus PHD 2000 syringepump. After the solution had been at 80° C. for approximately 10 min(solution temperature verified through use of a thermocouple) thestyrenic monomer was added to the solution at a rate of 0.02 mL/minuntil 2 mL of monomer had been added. The solution was stirred at 80° C.for approximately two hours after the monomer had been added. Thesolution was then cooled to room temperature and analyzed via dynamiclight scattering (DLS) for size and thermal gravimetric analysis (TGA)for solids content.

The instrument used for pyrolysis-GCMS experiments was a CDS 5250pyroprobe mounted to a GCMS outfitted with a 30M 35% phenyl column andusing electron ionization. 1 μL of sample was used per run.

Pyrolysis parameters:Drying stage—200° C. for 2 minPyrolysis stage—700° C. for 1 minTransfer line temperature—300° C.GCMS parameters:Carrier A control—Pflow-HeSplit ratio—50:1Front injector setpoint—300° C.Oven program

Initial Temp—40° C. Initial Hold—2.0 min

Ramp program—12.0° C./min to 300° C. hold for 10.0 minMSD parameters:Type—MS scanIon mode—EI+Start mass—35.00End mass—550.00Scan time—0.2 sec

FIG. 19 shows total ion chromatograms (TICs) after programmed pyrolysisof polymeric tracers in aqueous matrices: from top to bottom, the panelsshow polystyrene nanoparticles in deionized water, polystyrenenanoparticles in synthetic seawater, poly-t-butylstyrene nanoparticlesin deionized water, and poly-t-butylstyrene nanoparticles in syntheticseawater. The spectra containing styrene and t-butylstyrene aredistinct, demonstrating the barcoding capability of the polymerictracers.

The results in FIG. 19 demonstrate the following characteristics of thisapproach: (1) polymeric tracers can be cleanly decomposed into theirconstituent monomers at elevated temperatures and the resulting monomerscan be detected via GCMS; (2) programmable pyrolysis permits directdetection of polymeric tracers in complex matrices such as syntheticseawater which would otherwise be unsuitable for GCMS; (3) styrene andt-butylstyrene are easily differentiable by mass, thereby providingevidence of a rich barcoding scheme; and (4) no extraction or isolationsteps were performed on the particles, thereby eliminating any timeconsuming laboratory intervention.

To emphasize the barcoding capability of the approach, a mixture of bothpolymer particle platforms was prepared in the same synthetic seawaterbase fluid—not unlike what may occur in the field if two injectors arecommunicating with one producer. The heterogeneous sample was pyrolyzedand analyzed as in the previous example. The results are shown in FIG.20. FIG. 20 shows a total ion chromatogram after programmed co-pyrolysisof a mixture of styrene and 4-tert-butylstyrene nanoparticles. Note thations originating from both monomers show up in the chromatograms,thereby demonstrating the capability to uniquely identify variouspolymeric materials via pyrolysis-GCMS.

In summary, a polymeric tracer system, composed of various polymericcores, that is capable of cleanly decomposing into its constituentmonomers at specific temperatures has been developed. This approachallows the creation of a library of tracer particles that can beunambiguously detected and identified in highly heterogeneous media suchas produced water from oil fields. The synthesis and pyrolysis behaviorsof eight unique particle systems as well as mixed particles that containmore than one monomer have been demonstrated. Unambiguous detection ofthese particles in oily saline water has also been demonstrated.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of anyinvention or on the scope of what may be claimed, but rather asdescriptions of features that may be specific to particularimplementations of particular inventions. Certain features that aredescribed in this specification in the context of separateimplementations can also be implemented in combination in a singleimplementation. Conversely, various features that are described in thecontext of a single implementation can also be implemented in multipleimplementations separately or in any suitable sub-combination. Moreover,although features may be described as acting in certain combinations andeven initially claimed as such, one or more features from a claimedcombination can in some cases be excised from the combination, and theclaimed combination may be directed to a subcombination or variation ofa sub-combination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be helpful. Moreover, the separation of various system modules andcomponents in the implementations should not be understood as requiringsuch separation in all implementations, and it should be understood thatthe described program components and systems can generally be integratedtogether in a single software product or packaged into multiple softwareproducts.

Further modifications and alternative implementations of various aspectswill be apparent to those skilled in the art in view of thisdescription. Accordingly, this description is to be construed asillustrative only. It is to be understood that the forms shown anddescribed are to be taken as examples of implementations. Elements andmaterials may be substituted for those illustrated and described, partsand processes may be reversed, and certain features may be utilizedindependently, all as would be apparent to one skilled in the art afterhaving the benefit of this description. Accordingly, the description ofexample implementations does not define or constrain this disclosure.Other changes, substitutions, and alterations are also possible withoutdeparting from the spirit and scope of this disclosure.

What is claimed is:
 1. A method of tracing fluid flow in a subterraneanformation, comprising: providing a polymeric tracer comprising a polymernanoparticle having a polymeric core surrounded by a protectivepolymeric layer to an injector location in the subterranean formation,wherein the polymer nanoparticle comprises a surfactant layer betweenthe polymeric core and the polymeric layer; collecting an aqueous samplefrom a producer location in the subterranean formation; removing waterfrom the aqueous sample to yield a dehydrated sample; pyrolyzing thedehydrated sample to yield a gaseous sample; and assessing presence of apyrolization product of the polymeric core in the gaseous sample,wherein the presence of the pyrolization product in the gaseous sampleis indicative of presence of the polymeric tracer in the aqueous sample.2. The method of claim 1, wherein providing the polymeric tracercomprises providing the polymer tracer to the injector location to flowthrough the subterranean formation from the injector location to theproducer location, and wherein the polymeric layer comprisespolyethyleneimine (PEI) or a crosslinked carbohydrate-based coating. 3.The method of claim 1, comprising preserving the polymeric core inreservoir conditions in the subterranean formation via the polymericlayer, wherein the polymeric core comprises a vinyl polymer, and whereinthe surfactant layer comprises an anionic surfactant coated on thepolymeric core.